By Dr Chun Tang — MBChB (Manchester), MRCGP, MBA · Practising NHS & Private GP · Founder, Little Ox & MeCheck

Why Measuring Your NAD⁺ Level Is the Most Important Thing You're Not Doing for Your Health

I've been prescribing and advising on NMN supplementation for two years. In that time, the most consistent gap I see is this: almost nobody knows what their NAD⁺ level actually is.

They know they feel tired. They know their recovery has slowed. They've read about NAD+, listened to Sinclair or Huberman, and perhaps started taking NMN. What they don't know is their baseline — and without a baseline, supplementation is informed guesswork rather than precision health.

I co-founded MeCheck partly to address this problem. Not because I wanted to sell tests, but because the measurement is genuinely the missing piece in how most people approach cellular health.

What NAD⁺ Actually Is — and Why Its Decline Matters

NAD⁺ (nicotinamide adenine dinucleotide) is not a supplement or a trend. It is a molecule present in every cell of your body and involved in over 500 biochemical processes. It is the primary electron carrier in the mitochondrial electron transport chain — the process by which your cells generate ATP from food. It is the essential substrate for PARP enzymes that repair DNA damage. It is required by sirtuin proteins (SIRT1–7) that regulate cellular ageing, inflammation, and metabolic health. It regulates your circadian clock genes.

Without adequate NAD⁺, none of these processes operates at full capacity. And NAD⁺ levels decline — measurably, consistently, across virtually all species studied — from roughly your mid-30s. The decline averages 1–2% per year. By age 50, most people have approximately half the NAD⁺ they had at 25.

This matters because the biology of feeling older — the fatigue that sleep doesn't fix, the recovery that takes longer, the cognitive stamina that depletes by mid-afternoon, the metabolic changes that resist the same interventions that worked at 35 — maps almost precisely onto what we'd expect from reduced cellular NAD⁺ availability. The decline is not a metaphor for ageing. It is one of its measurable drivers.

The Measurement Problem — Why This Hasn't Been Done Until Now

If NAD⁺ is so important, why hasn't routine NAD⁺ testing existed? The honest answer is that until recently, it was technically very difficult. Measuring NAD⁺ accurately — and distinguishing it from NADH, its reduced counterpart — requires sophisticated analytical chemistry. Mass spectrometry, specialist reagents, carefully controlled pre-analytical conditions. It was not something that scaled to a consumer home test without significant innovation.

That barrier has now been cleared. Accurate NAD measurement opens the door to better clinical decisions, personalised wellness strategies, and more responsible product development — moving the field from assumptions to measurable biology. The NAD Level Test from MeCheck uses a dried blood spot collection technique that captures both NAD⁺ and NADH from a home finger-prick sample with the accuracy required for meaningful clinical interpretation.

Why Measuring Both NAD⁺ and NADH Matters

Most people discussing NAD+ testing focus on NAD⁺ alone. This misses something important. NAD⁺ and NADH exist in dynamic equilibrium within the cell — NAD⁺ accepts electrons to become NADH; NADH donates electrons back to become NAD⁺. The ratio between them (the NAD⁺/NADH redox ratio) is as diagnostically meaningful as either absolute value.

A low redox ratio indicates that NADH is accumulating relative to NAD⁺ — a sign of impaired mitochondrial electron transport and reduced cellular metabolism efficiency. This is associated with fatigue, metabolic dysfunction and increased oxidative stress. A high ratio with low absolute NAD⁺ indicates depletion rather than metabolic impairment. These are different problems with different implications for intervention.

Measuring only NAD⁺ is like knowing your fuel tank level without knowing whether your engine is running efficiently. The NAD Level Test gives you both.

Individual Variation — Why Your Number Is Specific to You

One of the most important things the research on NAD⁺ has established is the extent of individual variation. Two people of the same age, gender and lifestyle can have NAD⁺ levels that differ by a factor of two or more. The determinants include genetics, diet, stress history, recovery and overnight routine, exercise habits, prior illness, and the activity of NAD⁺-consuming enzymes like CD38 — which increases with age and chronic inflammation.

This individual variation has direct implications for supplementation. The person with critically depleted NAD⁺ is going to need — and benefit more from — NMN supplementation than someone whose levels are merely in the lower range of normal. The person who re-tests after 90 days of NMN and sees their levels unchanged may need to address CD38 overactivation, look at quercetin supplementation alongside NMN, or reconsider dose and timing. None of this precision is possible without a measurement.

The Case for Testing Before and After NMN Supplementation

If you are taking NMN, or planning to start, there is a compelling case for testing at two points:

Before starting: Your baseline establishes where you are. Are your NAD⁺ levels critically depleted — potentially explaining the fatigue and recovery issues you've been attributing to age or stress? Or are they mildly reduced? Or well-maintained despite your age? Each answer changes the clinical picture and the most sensible supplementation approach.

After 90 days of consistent supplementation: Re-testing after three months of daily NMN tells you whether the intervention has moved the needle. In clinical trials, NMN supplementation at 250–500mg daily consistently raises blood NAD⁺ levels — but there is variability. Some people respond strongly; others less so. Knowing your response allows you to make evidence-based decisions about continuing, adjusting dose, adding complementary interventions, or addressing other factors that may be limiting your NAD⁺ synthesis.

This is how longevity medicine actually works. Not "take this and hope," but "measure, intervene, measure again." The test creates the feedback loop that supplements alone cannot.

Who Should Test

The NAD Level Test is most clinically relevant for:

• Adults over 40 — where age-related NAD⁺ decline has been accumulating for 10–20 years and is most likely to be clinically significant
• Anyone taking NMN supplementation — to establish a baseline and measure response
• People with persistent fatigue, cognitive fog or slow recovery — where NAD⁺ depletion is a plausible contributing mechanism worth investigating
• Anyone approaching a period of high physiological demand — major life transitions, high-intensity projects, physical training blocks — where knowing your cellular metabolism status is clinically useful
• Anyone interested in precision health — who wants to move beyond "I feel better / worse" to actual biomarker-based feedback

The Cost of Not Knowing

The counterargument to testing is: "I'll just take NMN and see how I feel." This is understandable, and it may produce some benefit. But it has limitations.

Subjective improvement is difficult to attribute. People feel better for many reasons — placebo effect, seasonal changes, other lifestyle factors, sleep improvements. Without a baseline, you cannot know whether NMN specifically moved your cellular metabolism status. And if you're not feeling significantly better after 90 days, you don't know whether that means NMN isn't working, your dose is wrong, something else is limiting your NAD+ synthesis, or you were already well-maintained and didn't need supplementation in the first place.

The test is £249. NMN Plus is £9.99 a month. The test pays for itself in information if it answers even one of those questions definitively.

This article is for informational purposes only and does not constitute medical advice. The NAD Level Test is a laboratory health assessment, not a medical diagnostic. If you have health concerns or are taking prescription medication, consult your GP. NMN supplements are food supplements, not medicines.